Conventionally, in silver halide photographic materials, nucleating agents have been used for various purposes. For example, the hydrazines most frequently used as a nucleating agent have been used as a nucleus forming agent in internal latent image type direct positive silver halide emulsions and have been used for increasing the sensitivity and/or the gradation in negative type surface latent image forming silver halide emulsions.
Although there are many direct positive photographic processes, the process wherein previously fogged silver halide grains are exposed to light in the presence of a desensitizer and then are developed and the process wherein a silver halide emulsion having photographic nuclei mainly inside the silver halide grains is exposed to light and then is developed in the presence of a nucleating agent are most useful.
The present invention is directed to the latter process. Silver halide emulsions wherein photosensitive nuclei are present mainly in the silver halide grains and a latent image will be formed mainly within the grains are called an internal latent image type silver halide emulsion and are distinguished from silver halide grains wherein a latent image will be formed mainly on the surface of the grains.
Processes wherein an internal latent image type silver halide photographic emulsion is subjected to surface development in the presence of a nucleating agent to produce a direct positive image and photographic emulsions or photosensitive materials used in said processes are well known, for example, from U.S. Pat. Nos. 2,456,953, 2,497,875, 2,497,876, 2,588,982, 2,592,250, 2,675,318, 3,227,552, and 3,317,322, British Pat. Nos. 1,011,062, 1,151,363, 1,269,640, and 2,011,391, Japanese patent publication Nos. 29405/68 and 38164/74, and Japanese patent application (OPI) Nos. 16623/78, 137133/78, 37732/79, 40629/79, 74536/79, 74729/79, 52055/80 and 90940/80 (the term "OPI" as used herein indicates patent applications which have been laid-open for public inspection).
In the above processes of producing a direct positive image, the nucleating agent may be added to the developing solution, but it is more common to add the nucleating agent to the photographic emulsion layer or other suitable layer of the photosensitive material.
As a nucleating agent to be added to silver halide photosensitive materials, hydrazine compounds are the most well known and are described, for example, in U.S. Pat. Nos. 2,563,785, 2,588,982 and 3,227,552. However, when these hydrazine compounds are used by adding them to a photosensitive material, a substantially higher concentration (for example, about 2 g per mol of silver) of the hydrazine compound is needed and, further, since the nucleating agent diffuses from the emulsion layer to the developing solution during the development process, the concentration of the nucleating agent in the emulsion changes and image density becomes non-uniform. Further, in the case of multilayer color photosensitive materials, the diffusion of the nucleating agent from the emulsion layer breaks the balance of the nucleating action between the emulsion layers, thereby resulting in an imbalance between the colors in the developed image.
To obviate these defects, hydrazine type nucleating agents having a substituent that will be adsorbed on the surface of silver halide grains have been developed. As hydrazine type nucleating agents having an adsorption accelerating group, thiourea linkage type acylphenylhydrazine compounds described, for example, in U.S. Pat. Nos. 4,030,925, 4,031,127, 4,139,387, 4,243,739, 4,245,037, 4,255,511 and 4,276,364, and British Pat. No. 2,012,443 can be mentioned.
Further examples are compounds having an adsorption group such as heterocyclic thioamides described in U.S. Pat. No. 4,080,207; phenylacylhydrazine compounds having a heterocyclic group with a mercapto group as an adsorption group described in British Pat. No. 2,011,397B; sensitizing dyes having a substituent with a nucleating effect in the molecule structure described in U.S. Pat. No. 3,718,470; and the compounds described in Japanese patent application (OPI) Nos. 200230/84, 212828/84 and 212829/84, and Research Disclosure, No. 23510 (November, 1953), No. 15162 (Vol. 751, November, 1976) and No. 17626 (Vol. 176, December, 1978).
Generally, when hydrazine type nucleating agents are used, the difference between the maximum density (Dmax) and the minimum density (Dmin) is great and although the hydrazine type nucleating agents are most excellent from the point of view of the high contrast obtained, they are disadvantageous because a high pH (pH&gt;12) is required in development processing.
As nucleating agents that are effective even at a low processing pH (pH.ltoreq.12), heterocyclic quaternary ammonium salts such as those described in U.S. Pat Nos. 3,615,615, 3,719,494, 3,734,738, 3,759,901, 3,854,956, 4,094,683 and 4,306,016, British Pat. No. 1,283,835, and Japanese patent application (OPI) Nos. 3426/77 and 69613/77 are known.
Particularly, heterocyclic quaternary ammonium salt compounds substituted by a propargyl or butynyl group described in U.S. Pat. No. 4,115,122 are excellent nucleating agents in direct positive silver halide emulsions from the point of view of high contrast.
However, in silver halide emulsions, particularly in color photosensitive materials, sensitizing dyes are used for the purpose of spectral sensitization. Since competing adsorption onto the silver halide emulsion occurs between the sensitizing dye and the heterocyclic quaternary ammonium type nucleating agent, a quaternary salt type nucleating agent weak in adsorbability must be added in an amount greater than that which would otherwise be necessary to get the same result.
In particular, in the case of multilayer color photographic materials, sometimes the density becomes nonuniform or an imbalance occurs between the colors in the respective layers after development, and it cannot be said that the performance has been satisfactory with such materials.
To overcome these problems, a quaternary salt type nucleating agents having a thioamide AgX adsorption accelerating group has been suggested in U.S. Pat. No. 4,471,044. Although this patent discloses that the introduction of such an adsorption accelerating group decreases the amount of the nucleating agent needed and further prevents decrease of Dmax at high temperatures over time, the effect is not satisfactory.
On the other hand, although hemicyanine type quaternary salt compounds are described in Japanese patent application (OPI) No. 100426/82, they are not satisfactory with regard to nucleating activity.
It is known that processing a surface latent image type silver halide negative emulsion in the presence of a hydrazine type nucleating agent with a processing solution having a high pH (&gt;11) can produce a high contrast negative image high in gamma (&gt;10) as disclosed, for example, in U.S. Pat. Nos. 2,419,975, 4,224,401, 4,168,977, 4,243,739, 4,272,614 and 4,323,643.
U.S. Pat Nos. 4,385,108 and 4,269,929 disclose examples of hydrazine type nucleating agents having a group capable of accelerating dsorption on silver halide grains. It is also known that when a combination of a negative emulsion and a hydrazine compound is processed at a low pH (&lt;11), sensitivity can be increased.
In a silver halide negative emulsion system, it is known that quaternary ammonium type compounds act to accelerate development as described, for example, in U.S. Pat. No. 4,135,931, Japanese patent application (OPI) Nos. 114328/77 and 121321/77, German Pat. No. 2,647,940, and Belgian Pat. No. 721,568.